This invention relates generally to metals and metal alloys used in high temperature applications. In some specific embodiments, it relates to methods for treating superalloy components to prepare them for additional repair processes.
Superalloys are often the materials of choice for components intended for high-temperature environments. As an example, turbine blades and other parts of turbine engines (e.g., gas turbine engines) are often formed of nickel-based superalloys because they need to maintain their integrity at temperatures of at least about 1000° C.-1150° C. Protective coatings, often referred to as thermal barrier coatings or “TBC”s, effectively increase the operating temperature of turbine components by maintaining or reducing the surface temperature of the alloys used to form the various engine components.
It is common for gas turbine engine components to develop cracks, over the course of their operation. In many cases, the cracks originate on the surface of the component, and extend into the component. These cracks primarily arise over time, from extreme temperatures and pressures experienced by the turbine engines. Of particular concern are cracks which form in turbine engine blades, e.g., the high pressure turbine (HPT) blades closest to the engine combustor. These blades are subjected to the hottest combustion gases from which energy is extracted, and are most often formed from the nickel (Ni) superalloys. (These specialized alloys tend to produce equiaxed, directionally-solidified, and single crystal alloy structures).
FIG. 1 illustrates an exemplary turbine blade 10, for use in power generating turbines, e.g., the first row of blades of a gas or combustion turbine. Turbine blade 10 includes a blade root 12, an airfoil portion 14, and a tip portion 16. The blade root 12 is designed to be inserted into and retained by a disc on a rotating shaft (not shown) of the turbine. Airfoil portion 14 is shaped to extract energy from combustion gases passing over the airfoil portion 14, thereby imparting rotating mechanical energy to the turbine shaft. For modern gas turbine engines, airfoil portion 14 is designed to include one or more cooling passages formed below the surface of the airfoil for the passage of cooling air necessary to insure the integrity of the blade material in the hot combustion gas environment.
FIG. 1 provides an illustration of one of the cracks 24 that can develop near the tip 16 of the blade 10. As alluded to previously, the cracks can develop due to low cycle fatigue stresses imparted on the blade tip 16 during the operation of the turbine. If the crack 24 extends beyond a critical dimension, the turbine blade 10 must be removed from service and/or repaired, in order to prevent failure of the blade and turbine.
In many instances, the crack may be repaired by removing the material adjacent to the crack 24, to form a crack repair volume, and then filling the crack repair volume with weld metal. In general, a number of techniques are used to repair cracks. Several prominent examples include welding, diffusion brazing, activated diffusion healing (ADH), and thermal spray techniques, such as high velocity oxy-fuel (HVOF).
For most of these techniques, the initial preparation of the repair surface is critical. The undesirable presence of chemically-stable oxides, as well as any metallic bond-coat or ceramic thermal barrier coating material, can greatly impede welding or brazing of a repair surface. Thus, such materials must be completely removed, to allow for treatment, e.g., to allow for successful turbine overhaul.
Various methods have been used in the past to clean the repair surfaces of metal components like turbine blades. In some instances, manual grinding is carried out, prior to repair by welding techniques. While the grinding techniques are useful in many situations, they are also time-consuming. Moreover, the effectiveness of the grinding is very dependent on the skill of the operator. Furthermore, in the case of cracks within turbine airfoil surfaces, grinding procedures are often limited to a crack depth of about 0.35 inch (8.9 mm). Thus, deep cracks in components like HPT blades cannot always be successfully “prepped” for repair; and attempts to carry out the preparation process can sometimes result in “weld drop”.
Various fluoride ion cleaning (FIC) techniques have also been used to clean cracks and other cavity surfaces. (Some of the related techniques are referred to as the “Dayton Process”). The FIC processes usually rely on the thermal decomposition of a fluorine-based polymer such as polytetrafluoroethylene (PTFE). According to some mechanisms, the resulting carbon-fluoride monomers combine with hydrogen, and these products contact various oxide deposits, converting the deposits to fluoride compounds. The fluoride compounds are volatile, and leave the cavity area in a gas stream. Heating steps are often used to transform any remaining surface deposits and diffused deposits to volatile products which can be removed in gas form.
While fluoride ion cleaning and etching processes are effective in some situations, they also may exhibit significant disadvantages. As an example, these techniques can require relatively long process times if the workpiece surface is to be completely cleaned. They may also require high cleaning temperatures, e.g., about 1900° F. (1038° C.). Moreover, FIC processes often rely on the use of compounds such as hydrogen fluoride. These materials are corrosive and toxic, and require special handling and disposal procedures. Some of the compounds are also classified as hazardous air pollutants. Furthermore, the FIC process can attack the materials which form protective coatings on superalloy articles, e.g., nickel-aluminide or platinum-aluminide materials. Thus, care must be taken to mask or otherwise protect substrate areas where these protective coatings must be retained, e.g., areas away from the specific repair region.
Accordingly, new techniques for cleaning cavities and other regions in superalloy substrates would be welcome in the art. The techniques should be effective in removing oxides and other contaminants from the cavity region, as well as removing any protective coating materials, in preparation for repair processes. Moreover, the techniques should be capable of being carried out in a relatively short time period, and at ambient temperature. They should also minimize or eliminate the need for chemicals which are hazardous and require special safety procedures.